The invention relates to an alloplastic bone replacement that includes a length-adjustable part having a spindle and a sleeve that are locked against rotation.
If a long bone of a young person who has not yet attained his or her full height is completely or partially replaced by an implant, difficulties arise from the fact that the implant is unable to follow the growth of other parts of the body. For example, if the tibia or part thereof is replaced by the implant because of tumour invasion, this implant does not follow the growth of the fibula.
U.S. Pat. No. 5,358,524 discloses a length-adjustable implant in which a threaded spindle is inserted partially in a sleeve and the length of insertion is determined by a nut which can be screwed onto the spindle. By turning the nut, whose axial position relative to the sleeve is fixed, the length of insertion of the spindle can be changed and adapted to the individual requirements. Sleeve and spindle are provided with rotation-locking means comprising a longitudinal groove in the spindle and a pin located on the sleeve and engaging in this longitudinal groove. An axial securing arrangement, i.e. an arrangement preventing an axial relative movement between the spindle and the sleeve in the secured state, is formed by a locking ring arranged on the sleeve for rotationally locking the nut.
The object of the present invention is to simplify the structure of such a bone replacement and make it easier to configure.
According to the invention, this is achieved by the characterizing clause of claim 1 and preferably by the features of the subclaims. Accordingly, the axial securing arrangement is formed by the bottom of the groove provided for the rotation-locking and by the engagement part engaging in the groove, said engagement part being designed as a screw. The screw thus fulfills two functions: in a first position it engages only loosely in the groove and permits axial movement, whereas in another position it presses on the bottom of the groove and thereby effects the axial securing function. In contrast to the known solution, the locking ring for rotational locking of the nut can be entirely dispensed with. According to the invention, it is also possible to dispense with those components of the known device by means of which the nut is axially fixed relative to the sleeve. Dispensing with these elements results in a considerably simplified construction whose ease of configuration greatly improves its user-friendliness.
The axial fixing can be still further improved by the bottom of the groove having a series of depressions and by the screw engaging in each case in one such depression.
The nut too is preferably provided with rotation-locking means which can likewise be formed by a screw which engages in the longitudinal groove of the spindle.
The arrangement according to the invention is part of an implant which is connected at both ends to parts of the skeleton. For example, a stem for anchoring in the medullary cavity of a bone can be provided at one end or, if desired, at both ends of the arrangement. This stem need not be explained in detail, as stems for anchoring in bone cavities are known. Moreover, a joint component can be provided at one end or, if desired, at both ends, for example a knee joint component at one end and an ankle joint component at the other. In many cases, it will be preferable not to connect these joint components directly and non-releasably to the arrangement, but instead to provide coupling members at the ends of the arrangement on the one hand and on the joint components on the other, thus permitting different combinations. These couplings can be cone couplings, for example, which are known in the prior art.